Effect of dissolved oxygen concentration (microaerobic and aerobic) on community structure and activity of culturable heterotrophic nitrifying bacteria in activated sludge

To investigate the community structures and heterotrophic nitrifying activities of culturable heterotrophic nitrifying bacteria (HNB) in activated sludge, two sludge samples, cultivated at aerobic (2.5–4.0 mg·L−1 dissolved oxygen) and microaerobic conditions (0.5–1.0 mg·L−1 dissolved oxygen) respectively, were analyzed by isolation, purification, sequencing, and phylogenetic analysis. Ultimately, 48 and 50 effective 16S rDNA sequences (representing >80% of the total colonies on an agar plate) were collected from the two samples, from which seven and eight operational taxonomic units (OTUs) were obtained, respectively. All OTU-identified isolates showed typical heterotrophic nitrification, including six reported and six previously undescribed HNB species. The two culturable HNB 16S rDNA sequence libraries exhibited significant differences in the diversity and relative abundance of culturable HNB species. The microaerobic conditions encouraged a more diverse culturable HNB community in activated sludge compared to aerobic conditions.